Biopotential measurements, also known as bioelectric potential measurements, and other electrical measurements at the surface of the skin of a living organism are often degraded by undesired electrical noise, electrical pickup and motion artifact, which involve potentials which compete with the desired biopotential measurement. For example, electrocardiogram (EKG) measurements are widely used, but in situations such as exercise there is considerable tissue movement, and this causes competing, undesired voltages. In general, this type of measurement problem is most difficult when the resistance associated with skin is largest. Resistance-lowering techniques such as mechanical abrasion or adhesive tape stripping of stratum corneum of the skin can significantly improve the quality of such electrical measurements. However, the need to mechanically alter the skin is highly undesirable, as it can be difficult to control the degree of mechanical alteration, can cause pain and discomfort, and can lead to infection.
In the case of electroporation generally, strong electric field pulses applied to cells that cause the transmembrane voltage to exceed about 0.2 V for long (e.g. 100 ms) pulses, and about 0.5 V for short (e.g. 1 ms) pulses are well known to cause ionic and molecular transport across the cell membrane. More recently, in the case of short pulses that cause the transdermal voltage, U.sub.skin, to exceed about 50 V, ionic and molecular transport across the stratum corneum (SC) is also greatly enhanced. In both cases, the hypothesis is that aqueous pathways ("pores") are created by the applied electrical pulses such that greatly increased ionic and molecular transport occurs because of these pores. Summaries of what is understood about electroporation can be found in the book Guide to Electroporation and Electrofusion, Chang et al., Eds. (Academic Press) (1992), and in the reviews Weaver and Chizmadzhev, "Electroporation," in CRC Handbook of Biological Effects of Electromagnetic Fields, Second Edition, C. Polk and E. Postow, Eds. (Boca Raton: CRC Press), pp. 247-274 (1996), and Weaver and Chizmadzhev, "Theory of Electroporation: A Review," Bioelectrochem. Bioenerget., 41:135-160 (1996), the teachings all of which are incorporated herein in their entirety.
During a simulating pulse that causes electroporation, a large voltage exists across the biological barrier (the cell membrane in the first case; the approximately 100 lipid bilayer membranes of the SC in the case of the skin), and a greatly diminished electrical resistance occurs across that barrier. In many instances, the resistance returns to prepulse values, or nearly prepulse values, comprising "reversible electroporation." For the larger pulses, and for longer pulses, artificial planar bilayer membranes exhibit irreversible breakdown, and are destroyed, so that resistance across the site of the membrane remains at a greatly diminished value. Similarly, for the larger pulses, and for longer pulses, cell membranes remain in an open state, with a greatly diminished transmembrane resistance, and the cell is usually killed. In the case of skin, for the larger pulses, and for longer pulses, R.sub.skin can remain at values much smaller than the initial, prepulse values. This lack of recovery is often viewed as evidence of damage to electroporated cells, often fatal damage, in the case of the SC lack of recovery is often assumed to be undesirable, even though the SC is a dead tissue. Further, a lack of recovery means that the adjacent, viable epidermis is exposed through the persistent pathways to the external environment, i.e. some of the protective feature of the skin has been lost. Thus, against the background of artificial planar bilayer membrane electroporation and cell membrane electroporation, the use of large and/or long pulses that cause skin electroporation but with slight or essentially no recovery of R.sub.skin is viewed as undesirable.